Abstract
The electrolysis of (bi)carbonate solutions can upgrade captured carbon dioxide (CO2) into chemicals and fuels. This process relies on a pH swing to release CO2 from the sorbent near the cathode surface. Here, we report a bicarbonate electrolyzer equipped with an operando Raman spectroscopic probe that measures the pH at or near the cathode surface as a function of current density. We correlate this cathode surface pH with the selectivity of CO2 reduction products during the electrolysis of (bi)carbonate CO2 capture solutions at different bulk pH values. Our results show that C2 products, such as C2H5OH, are favored over C1 products, including CO, CH4, and HCOO–, when the cathode surface pH is high. However, at higher current densities, where the surface pH is highest, C2 product formation becomes less favored because less CO2 is available for C–C coupling. Consequently, the highest Faradaic efficiency for C2 product formation, 22%, occurs at the lowest current density measured, 100 mA cm–2.